In a known crank drive of this type, which has become known from German Pat. No. DT-PS 226734, and which includes a drive for a cold saw having a saw blade moving to and fro, there is provided a pivotable lever for adjusting the stroke of the saw blade in a hinge of the drawbar, which hinge joins the two portions of the drawbar, and is also supported within the saw on a rigid support, although the support is adjustable in its position. The fulcrum of the lever is adjustable along a circular track, the hinge being located in the center point of the track when the crank drive is fully extended, and the fulcrum may be arbitrarily set. Depending upon the adjustment of the fulcrum on a predetermined portion of the circular track, the crank drive will have a different stroke, which is larger than the stroke of the crank drive, if the lever is omitted. The position or location of one end of the stroke may be changed for the different stroke motions, while the position of the other end of the stroke, which is obtained upon the crank drive being fully extended, remains unchanged. In order to obtain an effective magnification of the stroke of the crank drive, the lever must be relatively long, and may not subtend too small an angle with the drawbars in an extended position of the crank drive. But this results not only in the fulcrum of the lever being disposed considerably outside of the space required by the gear itself, but by the drawbar itself moving along a circular arc, which lies considerably outside the radius of the crank. A lever arrangement of this type therefore not only requires additional gear parts, such as both a lever and a hinge disposed in a housing in an adjustable manner, so that the parts can be preset, but it is primarily relatively bulky and requires considerable availability of space to execute the required pivotable movement of the lever, and the parts of the drawbar.
In photographic cameras, there are employed for the winding of the shutter by the filter feed, for the winding of the mirror gear, or for operatively focusing the lens, crank drives of a conventional nature, namely everywhere where a rotational movement has to be translated into a linear movement; such crank drives include a crank having a crankpin, and a draw-or pull-bar pivoted to the crankpin. These conventional crank drives permit only a stroke of the order of the crank diameter upon the crank being rotated by 180.degree.. For a given stroke, the minimum diameter of the crank is therefore also given.
The current requirements demanded of a camera, particularly as its photographic and technical efficiency are concerned, and the additional need to construct such a camera in a compact manner, leave the designer of such a camera very little room to manipulate as far as the arrangement of mechanical construction parts is concerned, so that the required stroke frequently demands a crank diameter, which can no longer be accommodated within a volume available. In such cases the designer is forced to resort to additional transmission gears, which may, it is true, be disposed in another plane, but which after all require additional space, which may not be available, and which in any case render the drive much more expensive.
While it is possible, from a purely theoretical point of view, to transfer the principle of the aforesaid known crank drive of a saw blade and a cold saw device to the construction of a camera, such a transfer would fail because, as has already been stated earlier, the cold saw drive is not sufficiently compact, and requires a great deal of free space, which is not available in any camera.